Computing devices may include human interface devices (HIDs) that enable a person to create input data for, and in some cases sense output data from, a computing device. Computing devices may include, but are not limited to, personal computers (including both desktop and notebook computers), tablet computing devices, mobile handsets, personal data assistants (PDAs), kiosks, point of sale devices, or other personal entertainment devices (i.e., portable gaming devices, portable music and/or video players, etc.).
Some HIDs include a sensor surface. A sensor surface may detect the contact of an object on a surface, or the proximity of an object to the surface. Some of the many variations of a sensor surface include, but are not limited to: a touch sensor pad (i.e., touchpad) often used to emulate the function of a personal computer (PC) mouse, or a touchscreen often used as an interface for mobile sets, tablet computers and some personal computers. Sensor surfaces may provide one-dimensional sensing, detecting an object movement in one dimension (e.g., slider, single touch button, etc.), two-dimensional sensing, detecting movement along two axes, and may even include three dimensional sensing (sensing an object position in space proximate to the sensor surface).
Some sensor surfaces may operate by way of capacitance sensing utilizing sensor electrodes. A capacitance, as detected by sensor electrodes, may change as a function of the proximity of an object to the sensor electrodes. The object can be, for example, a stylus or a user's finger. In some devices, a change in capacitance may be detected by each sensor in the X and Y dimensions of a sensor array. According to detected changes in capacitance at such sensors, a position of an object (or objects) may be determined.
FIG. 14 shows one example of conventional “self” capacitance sensing. FIG. 14 shows a set of sensor electrodes 1400. A self-capacitance (Cp) of one selected electrode 1402 may be sensed by connecting such a sensor to sense connection (shown as RX). A self-capacitance (Cp) may be a capacitance of the selected electrode 1402 with respect to ground. If a sensed self-capacitance is outside of a threshold limit, an object may be considered present at the position of the selected electrode 1402. Conversely, if a sensed self-capacitance is within a threshold limit, an object may not be considered present at the position of the selected electrode 1402.
FIG. 15 shows one example of conventional mutual capacitance sensing. FIG. 15 shows a two set of sensor electrodes 1500. A mutual capacitance (Cm) may exist between two electrodes: a receive (Rx) electrode 1502-0 and a transmit (Tx) electrode 1502-1. It is understood that one set of electrodes (e.g., 1502-0 or 1502-1) may be disposed perpendicular to the other set of electrodes (e.g., 1502-1 or 1502-0). A periodic signal may be transmitted on the Tx electrode 1502-1. Due to mutual capacitance (Cm), the electrical signal at the Tx electrode 1502-1 may induce a current on the Rx electrode 1502-0. Similar to the conventional self-capacitance sensing of FIG. 14, if a sensed mutual capacitance is outside of a threshold limit, an object may be considered present at the position of the selected electrodes 1502-0/1. Conversely, if a sensed mutual capacitance is within a threshold limit, an object may not be considered present at the position of the selected electrodes 1502-0/1.